Witnessed a climber at fourth clip, who had z-clipped all the way from the bottom clip upwards to third clip (must have pulled up the belay side of the rope to clip twice, and somehow managed to climb on upwards).

If they had fallen, they would have hit the ground, as the climber end of the rope was into first clip (and then back up to second and third clip and back to belayer).

Both climber and belayer had not noticed, so I can only assume they didn't know what they were doing.

Shouted for the climber to climb down and they escaped without injury, for a long chat with wall staff.